Key equations for Process Engineers Process Control Process Design

Advanced control valve sizing guidelines

When sizing the control valve, caution needs to be taken for covering all possible scenarios and sometimes need to ensure some of the non-idealities like cavitation are dealt. Some of the problems faced are as follows

Big difference Cvmax and Cvmin

In many flow control valve cases, you might encounter situation where there is wide variations of flowrates. Typically rangeability of linear globe control valve is up to 1:5.

In such cases, use equal percentage vales which can give high variations in the control valves. Typically rangeability of equal percentage globe control valve is up to 1:20.

Extremely high pressure difference Cvmax and Cvmin

If the flow variations is extremely high and with one control valve with equal percentage characteristic it is not able to handle then following alternatives can be explored

  • If the flowrate variations happening in different period of operations like start of operation or end of operation, then valve replacement is one of the option.
  • If the variation happens only for short duration and upstream system has some buffer to accommodate the fluid without any major issues then such cases can be removed from control valve cases. Upstream system adequacy check and ultimate client's approval is critical in such decisions.
  • If the variation happens frequent and upstream system cannot accommodate the variations then split range control valve can be used. In this case, normal flow occur only one control valve and second valve will be open in case of sudden increase in the flowrate. Higher flowrate will be dealt by parallel control valve with bigger.

Critical or choked flow

Typically when downstream pressure is lower than half of the upstream pressure, the flowrate through the control valve remains same even though pressure downstream is reduced further for same (or maximum) opening, this type situation is called as critical or choked flow in control valves.   When pressure drop across the control valve is too big then Cv selection can play a big role and cautions needs to be taken when sizing control valve with high pressure drop or pressure of downstream too low.

High temperature drop or very low temperature downstream of valve

When dealing with hydrocarbon gaseous or multiphase systems, the pressure drop across control valve will lead to lowering the temperature due to Joule-Thomson effect. Sometimes the temperature can go down very low and material selection becomes a concern area. Control valve body, trim and other parts selection becomes crucial in such situations as well as need to access the possibility of hydrate formations. Low temperature issues can be dealt by increasing temperature upstream of control valve or selecting appropriate material of valve and downstream system.  Hydrate inhibitors can be injected to deal with hydrate formation issue.

Cavitation

Generally cavitation is referred for pumps and not for control valves. But for control valve also, cavitation can be big concern for liquid services. In the valve system you might have pressure downstream of control valve higher than vapor pressure but in the valve body the pressure can go lower than the vapor pressure of liquid and then recover above vapor pressure and engineer might miss the possibility of cavitation situation. The situation is explained in below figure

Control Valve Sizing

If the engineering or control valve vendor does not address this situation of cavitation then control valve might experience high vibration, noise as well as damage of valve. If the outlet pressure is close to vapor pressure and very high pressure drop, engineer need to check with valve manufacturer for possible cavitation and design trim suitable trim for possible cavitation.

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